Tanning



Patented June 25, 1940 UNITED STATES PATENT OFFIE TANNING No Drawing. Application June 16, 1938, Serial No. 214,050

26 Claims. (01. 149-5) This invention relates to the production ofv leather, It is more particularly directed to the tanning of skins with an acidic polymeric material in which the acidity is due wholly or principally to carboxyl groups which are attached to aliphatic carbon atoms, and to the leather produced. I.

The conversion of skins into leather is accompanied by profound changes in their physical characteristics. They become opaque, and a distinct fibrous structure becomes apparent. The stiffness characteristic of dry rawhides disappears to a great extent andthey become comparatively flexible. They assume a characterisl5 tic attractive appearance and soft feel. Tanning also renders the product imputrescible. For useful tanning these changes must be relatively permanent, and the leather should be resistant to de-tanning by water, at least at ordinary temperatures.

The ancient and well-developed art of tanning customarily effects the conversion of skins into leather by the use of vegetable preparations that contain tannins. Vegetable tanning materials,

such as quebracho extract, are quite slow in their action, and tanning processes employing them require excessively long periods of time for completion. The leather produced using vegetable tanning materials is characteristically colored,

and it is difiicult to obtain good true colors upon dyeing. It is particularly difficult to produce satisfactory light-colored leather.

Tanning processes employing metal compounds, such as chromium sulfate, are also known. Such processes are, however, subject to a number of disadvantages in operation, and the leatherproduced is characteristically colored. White leather, so-called, can be produced by employing bleaching if required, but this leather is not truly I white since it has a distinct blue, yellow, green,

or gray tint.

Numerous synthetic tanning materials have been proposed, but these are markedly inferior to the vegetable or metal compound tanning agents mentioned. In some instances the leather produced with known synthetic tanning agents is not sufliciently resistant to de-tanning by water, and in other cases the leather lacks the necessary fullness and plumpness. Therefore, these synthetic tanning materials of the prior art are customarily used in conjunction with a vegetable tanning material or a metal compound tanning agent, and they are useful principally because of properties apart from their characteristics as tanning agents.

It is an object of this invention to provide novel agents which may be used alone or in conjunction with known tanning materials to treat skins and to make leather. It is a further object of this invention to provide a fundamentally new type 5, of tanning process. It is a still further object to provide processes which offer a rapid, simple, and easily controlled method for the tanning of skins. It is a still further object of this invention toprovide tanning processes by means of 10 which there may be produced a good quality of leather. It is another object of this invention to produce white leather. Still further objects will become apparent hereinafter.

These and other objects of my invention are 15 attained by the use of an acidic polymeric material in which the acidity is due to carboxyl groupsattached to aliphatic carbon atoms, i. 9., carbon atoms which are not a part of an arc-- matic nucleus. In its preferred embodiment, the 20 invention deals with the use as tanning agents or acidic polymers of alpha-unsaturated aliphatic carboxylic acids. In the preferred processes of this invention, the acidic polymer is introduced into a skin, and then the combination of the skin 5 with the polymer is accelerated by lowering the pH of the system. The leather so produced is white to the extent of having no noticeable tint, and, when compared with the ofi-color leather produced by the prior art methods, it is a very 30 true white. The leather is additionally of good character, being opaque, soft, full, and flexible.

The polymers with which the present invention is concerned apparently form, probably thru their plex with skins, and I believe that the leather produced according to the invention comprises a reaction product of the acidic polymer with skin substance.

The expression acidic polymeric material in 40 which the acidity is due to carboxyl groups includes a wide variety of polymers all generically suited for use in the present invention. Included are, not only polymers in which the polymeric structure is built up by synthetic means, but also 45 thoseirl which the polymeric molecule is built up by nature, such as cellulose glycolic acid, alginic acid, and pectic acid. The suitable acidic synthetic polymers include, not only the selfpolymers and interpolymers of polymerizable ali- 50 phatic carboxylic acids, but also acidic condensation polymers, e. g., those in which an aliphatic polycarboxylic acid, such as citric acid or tricarballylic acid, is reacted with a polyhydric alcohoL-such as glycerol.

free carboxyl groups, some kind of chemical com- 5 methacrylic acid suited for use in the tanning of skins are discussed in application Serial Number 214,051, filed concurrently herewith. Still other suitable polymerization products of alpha-unsaturated aliphatic carboxylic acids are the interpolymers of acids of the acrylic series with poly-- merizable materials, for example, styrene and methyl methacrylate. Mixtures of such polymerlzation products may of course be used.

The term polymerization products of alphaunsaturated aliphatic carboxylic acids also includes interpolymers of alpha-unsaturated dicarboxylic acids such as maleic acid or of maleic anhydride, its equivalent, with polymerizable materials such as styrene. Products obtained by interpolymerizing the alpha-unsaturated carboxylic acids with each other such as interpolymers of methacrylic acid with acrylic acid or with maleic anhydride, may be used. There is no particular restriction on the type of added polymerizable material except that it be one which does not destroy the free acidity of the final polymer, to which I believe the tanning effect is largely attributable.

The present tanning agents are preferably used in aqueous media. The preferred agents are water-soluble initially, as is polyacrylic acid, or are capable of being brought into solution with aqueous alkali metal hydroxides, amines, or other bases, as are the interpolymers of malelc anhydride with styrene. For best results the solubility in either instance should be such that the solution may be brought to the proper pH for tanning without precipitation of the tanning agent. In some instances, however, agents that are moderately or even only slightly soluble can be used with some degree of success, due possibly to the fact that, as the small amount in solution is used up, a small additional amount of the polymer in turn goes into solution, and so on.

The polymerization products of this invention may be used for the treatment of any type of skin adapted to be converted into leather or fur. It is to be observed that the term skin is used generically to include heavy skins'such as cowhide, lighter skins such as goatskin and calfskin, and also skins of fur-bearing animals. It will naturally be necessary to adapt the processes to the type of skin to be treated. For instance, goatskin may be more rapidly tanned than calfskin, but those skilled in the art are well aware of such factors and may readily adapt the agentsof this invention to the specific needs of particular specialized problems.

Skins to be tanned according to the processes of this invention may be prepared in any of the manners customary in the art. They may suitably be unhaired, limed, delimed and bated, and pickled. The skins may if desired be de-pickled.

They may also be given a salt liquor treatment to soften them up and to render them of more uniform condition as is the practice in prior art tanning processes.

While skins prepared in any manner may be treated according to the processes of this inventain conditions of acidity and the skins may tend to change these conditions. The tanning solution may suitably be controlled to compensate for any variance of the pH of the skins from that of .the tanning solution. ,It may sometimes be found advantageous to adapt the processes for preparing the skins so that a skin prepared for tanning is approximately at the pH of the tanning solution.

Too rapid reaction of the tanning agent with the skin in the first stages of the process should be avoided in order to prevent surface hardening of the skin. In tanning with the agents of the present invention this result is accomplished by careful adjustment of the pH of the tanning solution. The exact acidity of a tanning solution containing a polymer according to this invention may be widely varied depending upon the specific type of skin to be treated and depending upon the specific type of leather to be produced. While a wider range of acidity may be used, it will generally be found desirable to have the tanning solution initially at a pH from about 4.8 to 5.7. Still more specifically, it will be found that under many conditions of operation a pH of about 5 is optimum. At a pH much below 5 the ailinity of skins for the polymerization products becomes so great that surface hardening takes place, while at a pH much above 5 the afiinity of the hides for the tanning agent is so decreased that penetration is not sufficiently rapid for the best results.

The acidity of the polymerization products may suitably be adjusted by the use of such alkalies as alkali metal hydroxides or carbonates, ammonium hydroxide, and organic bases such as ethanolamines. As has been observed before, the pH of the hide should be considered in adjusting the pH of the tanning solution so that the tanning solution will be employed at an acidity within a desired range. To assist in maintaining the tanning solution at about the optimum acidity a suitable buffer salt such as sodium acetate or sodium phosphate may be included in the tanning solution. It will be understood that the polymerization products may be sold to the trade in the form of their salts with for instance alkali metals or ammonia, or in the form of mixtures of such salts with unneutralized polymer. After satisfactory penetration of skins by the tanning agent has been effected the rate of tanning may be accelerated by lowering the pH of the solution. The pH to which the solution should be brought to effect complete tanning will vary considerably with the specific type of skin being treated. It will not generally be desirable to go to a pH below about pH 3 since, at a higher hydrogen ion concentration, hydrolysis of the skin may take place. As a practical matter it will be found that the pH should be lowered to from about 3.4 to 4.2, while more specifically it will be found that in-many processes a pH of about 4 will be most satisfactory.

The lowering of the pH of the tanning solution may suitably be effected by the use of any acidic material such as lactic, hydrochloric, sulfuric, acetic, phosphoric, sulfamic, and formic acids.

The considerations as to the type of apparatus to use, the time to be allowed for tanning, the temperatures of tanning, and the amounts of tanning agent are substantially those known to the art in connection with prior art tanning processes. The time allowed for tanning must be sufficient to permit complete penetration of the tanning agent and the pH should not be lowered too rapidly or there may be 'puckering and apparent over-tannage of the grain surface. The tanning can best be effected at room temperature, say to 30 0., but temperatures somewhat higher than those used by the art can successfully be employed if desired. The exact amount of the tanning agent to use can best be determined by a few simple tests under the specific conditions of the process selected and with the specific skin to be treated. In general, enough of the tanning agent must be used to effect the tanning desired but unduly large amounts should not'be used as they will not be taken up by the leather in a reasonable period of time.

The leather obtained according to the preferred processes of this invention is white, but it may if desired be treated with white pigments or may be colored during or after the tanning process by the inclusion of any of the common leather dyes and pigments.

While the acidic polymers of the present invention, and particularly, the polymers and interpolymers of alpha-unsaturated aliphatic carboxylic acids, are valuable tanning agents in their own right, they may if desired be used together with any other tanning agent or tanning assistant heretofore known to the art. They may, for instance, be used in conjunction with vegetable tanning materials such as quebracho extract, with chrome tans, or with synthetic tanning agents containing sulfonic acid groups. Additional "fullness or plumpness may be obtained by treatment with aluminum salts, such as aluminum sulfate.

Leather produced according to the processes of this invention may be subjected to any of the various kinds of finishing treatments customarily used. The leather if desired may be suitably filled or given any of the numerous surface treatments customary to the art.

My invention will be better understood'by reference to the following illustrative examples. 'Parts given are by weight.

EXAMPLE 1.--Polyaerylic acid-heat polymerized One part of acrylic acid was heated at 130 C. with a trace of benzoyl peroxide catalyst. When a vigorous polymerization reaction had begun, as shown by rise in temperature and refluxing, seven further parts of acrylic acid were slowly added in small portions. The reaction mixture slowly set to a solid mass. Heating was continued for four hours, until the polymer became rubbery in character. The product was readily soluble in water.

The heat-polymerized polyacrylic acid prepared as above described was used as a tanning agent in the following manner:

A tanning solution was made by dissolving three parts by weight of the heat-polymerized polyacrylic acid in 100 parts of water and adjusting the pH of this solution to 6.0 by addition of suitable amounts of sodium hydroxide. This solution was placed in a tanning drum, and there was added twenty-five parts of drained goatskin which had previously been pickled with a solution containing 1.5 per cent sulfuric acid and 10 per cent salt, and then de-pickled with sodium acetate and washed. The skins were drummed for two hours and allowed to stand in the tanning solution for an additional sixteen hours without agitation. Small amounts of a 10 per cent solution of lactic acid were then added with drumming between each addition until the pH The skins werethen a commercial fatliquor,

had been lowered to 3.7. washed, fatliquored with and dried.

It was found that the goatskins had been converted into a soft white leather of good quality.

Exlmru: 2.Polyacrylic acid-toluene polymerized Ten parts of acrylic acid and 0.1 part of benzoyl peroxide were dissolved in thirty parts of toluene and heated until a very violent polymerization reaction occurred at the boiling point EXAMPLE 3.H1/dr0lyzed methyl acrylate polymer A solution of 38.6 parts of methylacrylate polymer in 59.4 parts of toluene was heated to the boiling point in a vessel equipped with a reflux condenser. A solution of 22.5parts of potassium hydroxide in90 parts of absolute ethyl alcohol was then added in small portions. The reaction mixture became viscous. After refluxing for one hour, a white powder had separated. The solution was filtered and theproduct air dried. It was water-soluble.

The product preparedfrom methyl acrylate resin was used as a tanning agent by a procedure which differed from 1 and 2 for heat and toluol-polymerized materials only in that five parts by weight of the polyacrylic acid was used instead of three parts. The leather obtained was not quite as white or as soft as that obtained with the heator toluolpolymerized acrylic acid, but was nevertheless a satisfactory grade of leather.

ExAurLs 4.-Acrylic acid-methacrylic acid interpolymer A mixture of 0.15 part of benzoyl peroxide, 7.5 parts of acrylic acid and 7.5 parts of methacrylic acid was dissolved in 85 parts of toluene. The solution was placed in a flask equipped with a reflux condenser and heated to 95 C. A very vigorous polymerization reaction started at this temperature. The temperature slowly rose to 98 C. without further heating. After standing for thirty minutes to complete the reaction, the solution was filtered and the voluminous white powder obtained as product was air dried. The yield was thirteen parts. The product was soluble in water.

A tanning solution was made by dissolving three parts by weight of this polymer in one hundred parts of water. The pH of this solution was 2.8 and was adjusted to pH 6 by addition of sodium hydroxide. A

To this solution contained in a tanning drum was added twenty-five parts by weight of drained goatskins which had previously been subjected to pickling and de-pickling as described in Example 1, together with fifty parts of five per cent sodium chloride solution. The skins were drummed for three hours, and lactic acid addithat described in Examples EXAMPLE 5.-Maleic anhydride-styrene inter polymer A mixture of twenty parts of styrene and twenty parts of maleic anhydride was heated cautiously in an open vessel on a water bath. A very vigorous reaction occurred at 80 C., as evidenced by a sudden rise in temperature to 110 C. The reaction mixture was cooled and the brittle yellowish resin obtained as product purified by dissolving in acetone, and reprecipitating in a large amount of water. The product was isolated by filtration and dried in an oven overnight at 100 C. A yield of thirty-nine parts of polymer was obtained. The product was soluble in dilute aqueous alkali.

The maleic anhydride-styrene interpolymer, prepared as above described, was used as a tanning agent in the following manner:

A tanning solution was made by dissolving five parts by weight of the maleic anhydride-styrene interpolymer in two hundred parts of water, the rate of solution being accelerated by the addition of a small amount of concentrated ammonia.

When the interpolymer had dissolved, thepH was Y and there was added twenty-five parts of drained calfskin which had previously been pickled with a solution containing 1.5 per cent sulfuric acid and 10 per cent salt, and then de-pickled with sodium acetate, and washed. The skins were drummed for one-half hour, after which small amounts of lactic acid were added at intervals with drumming between each addition until the pH had been lowered to 3.7. The skins were then washed, fatliquored with a commercial fatliquor, and dried.

It was found that the calfskins had been converted into white leather of good quality,

As is apparent from the foregoing examples, the acidic polymers suitable for use in the present I invention may be made in a number of ways. The polymerization may be effected by any of several well-known procedures, such as at elevated temperatures with benzoyl peroxide as a catalyst and in the presence of solvent or dispersion media such as acetone, toluene, or water,

tions. It is known, for example, that in the polymerization of acrylic acid the temperature at which the reaction is carried out markedly influences the molecular weight of the polymer. As shown by Staudinger, comparatively low tem perature reactions tend to produce high molecular weight products whereas high temperatures have the opposite effect. In the work leading to the present invention it was found that these relationships were correct and that the tanning results vary somewhat with the different types of polymers. For example, when polymerization of acrylic acid was effected at room temperature by exposure to ultraviolet radiation, the product was a glassy,'brittle, solid which was diflicult to dissolve in water. This agent tanned only slowly and yielded a product lacking in opacity and fullness. Polymerization in a low-boiling solvent such as acetone, at a temperature of C. or less, also yielded an agent which produced relatively inferior leather. The poorer tanning effect of these high molecular weight polymers may be due to the relatively poorer penetration of the polymer into the skin.

On the other hand, as shown in the examples, polymerization by heating to high temperatures without a solvent or in a high-boiling solvent such as toluene at a temperature above 100 (2., yielded readily water-soluble products which produced good leather. Whereas these superior materials were polymerized to high molecular weight as shown by the fact that they are amorphous solids, their ready solubility in water indicated that they were much lower in molecular weight than the agent produced for example by polymerizing acrylic acid at room temperature by means of ultraviolet light.

It is to be understood of course that the above applies chiefly to acrylic acid and merely illustrates one of several instances in which somewhat difierent tanning results may be expected with polymers prepared by different methods.

In addition to the-polymers of the examples, the following polymerization products are illustrative of products suitable for use according to the present invention. All of these have been testedin accordance with the methods given in the examples, and in each instance white leather of good quality was produced.

The proportions given in parenthesis in the table refer to proportions of the constituents found in the polymers upon analysis; the other figures given under the heading Materials polymerized indicate the initial proportions of the starting material.

Polymerization products of alpha-unsaturated acids Materials polymerized Method of preparing polymer 855522 or Solubilities Alpha-ethyl acrylic acid Monomer exposed to ultraviolet light for five Opaque white solid Soluble in dilute sodium hydroxide.

ays. Acrylic acid styrene 25 15% solution of monomers in toluene at 100 C. White powder D0.

(82: 18). for 30 mins. with 1% benzoyl peroxide. Alpha-ethyl acrylic acid 83: Monomers exposed to ultraviolet light for 4 do Do.

styrencl7. days. Methacrylic acid 50: maleic 15% solution of monomers in toluene at 100 C. .....de Soluble in water.

anhydride 50 (92: 8). for 14 hrs. with 1.25% benzoyl peroxide. Malcic anhydride 50: rnctha- 15% solution of monomers at 90-100 C. for do Soluble in dilute alkali.

crylic acid 25: styrene 25. 4 hrs. with 1.25% benzoyl peroxide.

or by heating in the presence of a solvent and in the presence or absence of a catalyst, or by exposure to ultraviolet light.

Tanning action is affected to a certain extent by degree of polymerization and the latter in turn, as is well known, by polymerization condi- Also suitable for use in the present invention are the acidic polymers obtained by partial or complete hydrolysis of polymers or interpolymers of derivatives of alpha-unsaturated acids, such as the esters, amides, or nitriles. The use of one such polymer is illustrated in Example 3.

While I have shown certain specific illustrative processes and compositions, it will be understood that without departing from the spirit of my invention one skilled in the art may readily devise numerous tanning solutions and processes employing acid polymers of the type herein defined.

I claim:

1. In a process for the tanning of skins, the step comprising subjecting a skin to the action of an acidic polymeric material in which the acidity is due to carboxyl groups attached to aliphatic carbon atoms.

2. In a process for the tanning of skins, the step comprising subjecting a skin to the action in an aqueous medium of a polymerization prodnot of an unsaturated aliphatic carboxylic acid.

3. In a process for the tanning of skins, the step comprising subjecting a skin to the action of a polymerization product of an alpha-unsaturated aliphatic carboxylic acid.

4. In a process for the tanning of skins, the step comprising subjecting a skin to the action of a polymerization product of an acid of the acrylic acid series.

5. In a process for the tanning of skins, the step comprising subjecting a skin to the action of an interpolymer of members of the acrylic acid series.

6. In a process for the tanning of skins, the step comprising subjecting a skin to the action of polyacrylic acid.

7. In a process for the tanning of skins, the step comprising subjecting a skin to the action of a co-polymer of acrylic acid and methylacrylic acid.

8. In a process for the tanning of skins, the step comprising subjecting a skin to the action of a co-polymer of maleic anhydride and styrene.

9. In a process for the tanning of skins, the step comprising subjecting a skin to the action of a polymer of the type set forth in claim 1 at a pH of about 4.8,to 5.7 and then increasing the acidity.

10. In a process for the tanning of skins, the step comprising subjecting a skin to the action of a polymerization product of an unsaturated aliphatic carboxylic acid at a pH of about 4.8 to 5.7 and then increasing the acidity.

11. In a process for the tanning of skins, the step comprising subjecting a skin to the action of a polymerization product of an alpha-unsaturated aliphatic carboxylic acid at a pH of about 4.8 to 5.7, and then increasing the acidity.

12. Leather which comprises a reaction product of a polymer of the type set forth in claim 1 and skin substance.

13. Leather which comprises a reaction product of skin substance and a polymerization product of an unsaturated aliphatic carboxylic acid.

14. Leather which comprises a reaction product of skin substance and a polymerization product of an alpha-unsaturated aliphatic carboxylic acid.

15. Leather which comprises a reaction product of skin substance and a polymerization product of an acid of the acrylic acid series.

16. Leather which comprises a reaction product of skin substance and an interpolymer of members of the acrylic acid series. I

17. Leather which comprises a reaction product of skin substance and polyacrylic acid.

18. Leather which comprises a reaction product of skin substance and a co-polymer of acrylic acid and methacrylic acid.

19. Leather which comprises a reaction product of skin substance and a co-polymer of maleic anhydride and styrene.

20. A tanning composition comprising a partially neutralized acidic polymeric material in which the acidity is due to carboxyl groups attached to aliphatic carbon atoms, the tanning composition upon dissolving in water forming a tanning solution with a pH of about 4.8 to 5.7.

21. A tanning composition comprising a partially neutralized polymerization product of an alpha-unsaturated aliphatic carboxylic acid, the tanning composition upon dissolving in water forming a tanning solution with a pH of about 4.8 to 5.7.

22. A tanning composition comprising a partially neutralized polymerization product of an acid of the acrylic acid series, the tanning composition upon dissolving in water forming a tanning solution with a pH of about 4.8 to 5.7.

23. A tanning composition comprising a partially neutralized interpolymer of members of the acrylic acid series, the tanning composition upon dissolving in water forming a tanning solution with a pH of about 4.8 to 5.7.

24. A tanning composition comprising a partially neutralized polyacrylic acid, the tanning composition upon dissolving in water forming a tanning solution with a pH of about 4.8 to 5.7.

25. A tanning composition comprising a partially neutralized co-polymer of acrylic acid and methacrylic acid, the tanning composition upon dissolving in water forming a tanning solution with a pH of about 4.8 to 5.7.

26. A tanning composition comprising a partially neutralized co-polymer of maleic anhydride and styrene, the tanning composition upon dissolving in water forming a tanning solution with a pH of about 4.8 to 5.7.

GEORGE D. GRAVES. 

